[Sync] Finch Experiment : Enable compression between sync client and server

testing/variations/fieldtrial_testing_config_win.json

 {
     "BrowserBlacklist": [
         {
             "group_name": "Enabled"
         }
     ],
     "ChildAccountDetection": [
         {
             "group_name": "Disabled"
         }
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         },
         {
             "group_name": "ShowAndPossiblySend",
             "params": {
                 "sendingThreshold": "abcdef"
             }
         },
         {
             "group_name": "DontShowAndDontSend"
         }
+    ],
+    "SyncHttpContentCompression": [
+        {
+            "group_name": "Enabled"
+        },
+        {
+            "group_name": "Disabled"
+        }
     ]
 }